Despite being one of the most common illnesses of older cats, there is still a lot to be learned about chronic kidney disease. Early disease detection and staging is becoming increasingly important due to the ability to institute management and renoprotective strategies prior to the onset of clinical disease. While chronic kidney disease (CKD) in clinical patients is often relatively straightforward to diagnose, many cases are more borderline especially when concurrent diseases exist. The development of new tools for the diagnosis and staging of renal disease in cats recently has included blood tests such as symmetric dimethylarginine, urine tests (the ongoing search for renal biomarkers), and others.

As ultrasonography becomes more widely available in veterinary medicine, new techniques to investigate internal disease are becoming more prevalent. Investigation of kidney disease is one of these areas, where ultrasonography can determine the cause of many structural diseases and allow for more invasive testing such as aspiration or biopsy. The renal arterial resistive index (RI) is an ultrasonographic measurement calculated by determining the ratio of change in blood flow from systole to diastole to the systolic velocity. It is calculated by the equation:

This unit-less measurement is a factor of vascular resistance and compliance. It may be calculated from any renal artery (renal, interlobar, or arcuate) and is increased by high vascular resistance, low vascular compliance, or renal parenchymal lesions. In humane medicine it has been used since the 1970s, and while it cannot discriminate between causes of renal disease it does act as a marker for the presence of CKD.

The purpose of this study was to assess the utility of renal RI measurement in cats and its correlation with various factors associated with CKD such as age, sex, body weight, BUN, potassium, creatinine, IRIS staging, urine specific gravity, urine protein:creatinine ratio (UPCR), and systolic arterial pressure.

The study was designed as a prospective observational study. 24 cats were recruited from a population of animals presented to a veterinary hospital in Portugal. Cats on medications or with concomitant disease were excluded from analysis. Six healthy cats were used as controls and 18 had clinical signs of CKD. Two cats were IRIS stage 2, eight cats were stage 3, and eight cats were stage 4.

Renal RI was determined at the level of the renal artery in both kidneys. The cats were examined in lateral recumbency without sedation using Color Doppler to locate vessels and then Spectral Doppler to measure flow rates. Three measurements were recorded for each kidney and the mean was used in statistical analysis. Measurement was possible in all healthy cats and in 17 of the 18 (94%) diseased cats. One diseased cat had renal changes too severe to allow normal evaluation of architecture.

Renal RI was significantly higher in cats with CKD than in control cats for both kidneys. The only value that correlated with RI was body weight, for which lower weights were associated with higher RI. No difference was seen between RI in each kidney, and in fact the values were strongly positively correlated. While there was a tendency towards a higher RI in IRIS 4 cats, this was not statistically significant.

Receiver-operating characteristic analysis (a technique used to develop and assess cut-points for a test) was used to determine that a renal RI of 0.693 offered a sensitivity and specificity of 100% for diagnosis of CKD.

A major drawback to this study is the small sample size. No IRIS stage 1 cats were included and only two cats were IRIS stage 2. This may bias the sample by including primarily those cats with more severe disease and artificially increasing the sensitivity of the test. Further, only 1 cat had severe hypertension.

While renal resistive index calculation may not provide all the answers for feline CKD, it may prove to be a valuable additional tool for veterinarians. RI may allow for determination of the presence of CKD in asymptomatic cats and may have use in assessing for early changes in renal function. Further work should be done to investigate the utility of this technique in more varied situations and with larger numbers of cats. (MRK)